The use of the selective cold air drainage as a method of crop frost protection has been increasing in the world, and in particular in the US, since the 90's due to the growing application of the Selective Inverted Drain (SID). This device is protected by U.S. Pat. No. 5,647,165, of applicants authorship.
The SID can be applied in very different ways to reduce the damage produced by the radiation frost to diverse crops. The application diversity arises mainly from the topography of the area to protect. Frequently, orchards are located on large valleys with very gentle slopes (approx. 1/1000).
During frost nights (clear and calm nights with low relative humidity), cold air accumulates in these valleys, producing damage to the crops. Cold air accumulation areas area characterized by a stable and stratified atmosphere (density decreases with height and temperature increases with height).
To protect these orchards with SIDs, it is necessary to surround the area to be protected with artificial barriers (or curtains) many meters in height. In that way the SIDs drain the cold air in the protected area and the artificial barriers (located on the perimeter of the mentioned area) prevent the coldest and densest layers from entering the area up to the height of the artificial barrier. The combined action of the selective drainage and the barrier produces a higher temperature at canopy level within the protected area compared to the same height outside the protected area.
The construction of the curtains, with heights reaching several meters, is an important practical problem in the application of SIDs. Required heights range from 1.5 m to 8.00 m, depending on different factors that are analyzed in a case by case basis.
A common way to construct the mentioned curtains is to use fast growing vegetation with a foliage thick enough to block the flux of cold air. The main problem with this alternative is the long time (many years) needed to reach the desired heights; it is a long term solution to a problem that needs to be solved in most cases within few weeks or months.
Another solution consists of constructing the curtains with light, low cost plastic materials, supported by a steel tube structure, kept in a vertical position by means of turnbuckles secured to the ground. The advantage of this constructive method is that the entire curtain can be set in few days. The main problems of this kind of curtain are: vulnerability to strong winds, visual obstruction of the natural environment, the need to obtain special permissions to set the curtain, and the obstruction the curtain may cause to the free circulation of vehicles in the orchard. Besides, these light curtains demand maintenance during the frost risk season and a continuous supervision during frost nights.
It must also be noted that, occasionally, the sole use of natural or artificial curtains, with no SID, may constitute a very effective way of protection against the damage produced by radiation frost. The strategic location of natural or artificial curtains in the protected orchard can divert the cold air currents that would otherwise invade the orchard.
The construction of barriers free from the problems associated with natural curtains and light materials artificial curtains is of great practical interest for a range of solutions to control the radiation frost that are based on the modification of the cold air circulation over the orchard.
The above arguments have highlighted the need, within the radiation frost protection field, to substitute curtains of diverse materials by pneumatic barriers able to dam the cold and dense air away from the protected area. This need has been amplified by the increasing application of the device protected under U.S. Pat. No. 5,647,165.